This invention relates to the field of spray or aerosol delivery systems and, in particular, to providing a nonflammable propellant for such system.
Airless canister and aerosol systems offer the advantages of portability, ease of cleaning and convenience over more conventional spray systems. However, strict Department of Transportation (DOT) regulations govern maximum allowable canister shipping pressure so propellants must be used that can both aerosolize the contents into a useful spray pattern and maintain sufficient pressure to deliver the contents until the canister is empty.
Typically, one of the propellant components is a compound that at canister pressure can support both liquid and vapor phases. As the canister is emptied, vapor accompanies the adhesive out of the canister but the vapor is replenished from the liquid phase to maintain the equilibrium. It is more efficient to have most of this component in the liquid phase to conserve volume but this component must be able to vaporize rapidly when the liquid/vapor equilibrium is disturbed by the spraying operation. Low boiling point liquids that are commonly used for this purpose consist of but are not limited to dimethyl ether, liquefied petroleum gas blends such as A-70T, and hydrofluorocarbon and fluorocarbon blends such as Dymel® 152A, Dymel® 134A.
The second propellant component is usually a gas, included to provide a high enough pressure to empty the canister without compromising the spray pattern. Typically, nitrogen, air, (NO)x or carbon dioxide is used although in theory almost any gas can be substituted. An inert gas is preferred; oxygen, air and (NO)x can prematurely age certain formulation components, for example.
There is much art to the design of the propellant. The propellant components must be compatible with each other. Moreover, they must interact with the formulation in such a way so as to prevent phase separation of the liquid portion. Finally, some solubility of the propellant in the formulation is desirable so that when the material exits the spray gun, release of the soluble propellant provides the desired breakup of the liquid stream and uniform fan pattern.
Environmental considerations are becoming more and more important in shaping the composition of propellants. Thus, the formulator must consider the impact of using toxic, flammable, volatile organic compounds (VOC) content and hazardous air pollutants (HAPS). Insurance agencies are increasing fire insurance premiums for manufacturers storing flammable substances. Dymel® 152A and Dymel® 134A are the only propellants in the above list that are VOC exempt; of these, only Dymel® 134A is nonflammable. Neither Dymel® 152A nor Dymel® 134A is very compatible with most organic chemical-based formulations. Chlorofluorocarbons have been shown to damage the ozone layer and have been banned by international agreement.
There are very few propellant choices remaining if flammability and VOC issues are to be considered. It is therefore important to develop novel and innovative approaches to identify safe, nonflammable, VOC compliant and effective propellants.
Patent literature teaches the importance of formulation in promoting sprayability of water-based pressure pot systems. In U.S. Pat. No. 4,384,661, Page et al. disclose the use of a high hydrophilic/lipophilic balance (“HLB”) value nonionic surfactant to improve paint aerosolization by reducing foaming. Alcohol additives and polymer mixtures are used in U.S. Pat. No. 4,420,575 (Rapaport et al.) and U.S. Pat. No. 4,265,797 (Suk) to improve sprayable paint systems. Evans et al. (U.S. Pat. No. 4,477,613) stabilize an aqueous-based tackifier using a combination of small amounts of a nonionic or anionic surfactant and an elastomeric latex. Horwat et al. (U.S. Pat. No. 4,004,049) improve latex spray adhesion by introducing a controlled instability. In U.S. Pat. No. 5,444,112, Carnahan teaches use of a nonionic neoprene latex to aerosolize a water-based adhesive.
Equipment is also important in providing an even, consistent spray in water-based systems. Hammarth et al. (U.S. Pat. Nos. 6,905,084 and 6,848,599) describe the guns, hoses, canisters and accessories in some detail. In U.S. Pat. No. 5,931,354, Brand et al. claim an application system for sprayable water-based adhesives.
The focus of the present invention is the utility of certain propellant mixtures for both water-based and solvent-based airless canister and aerosol systems. Haraguchi et al. (U.S. Pat. No. 5,280,061) teach that “the solvents and the propellant are required to have mutual solubility or stable dispersion properties under sprayable pressure” and recommend dimethyl ether “from the point of view of mutual solubility and sufficient fluidity.” In EP 0 616 018 B1, Nguyen recognizes the importance of ecologically sound propellant systems while allowing that the mixtures of propellants used should be compatible with the other components of the formulation, but focuses on compatibilization by hydrogenation of the SBS block. Here also, the distinction is made between the liquid and gaseous components of the propellant system and the value of each is briefly discussed. Purvis et al. (EP 1 160 179 A1) claim design parameters for aerosol cans while teaching that a combination of liquid and gas propellants involving “one or more components” is “a preferred embodiment.”